^ v R22A0* 



, 



THE. 




INTERNATIONAL 

CORRESPONDENCE 

SCHOOLS. 

SCRANTON, PENNA. 
o 

Instruction Paper. 

SECOND EDITION. 



SUBJECT 



Mechanical 'Drawing. 



210 



2nd COPY, 




I 

NOTICE. — The Student must, in accordance with his 
agreement, treat this Paper as confidential. 



Entered according to the Act of Congress, in the year 1898, by The Colliery 

Engineer Company, in the office of the Librarian of Congress, 

at Washington. 



1964 



TMP96-Q24421 



Mechanical "Drawing. 



DIRECTIONS FOR DRAWING- PLATES. 



DRAWING PLATE, TITLE: DETAILS. 

Note. — All article numbers and figure numbers which occur in the 
text, are continued from Instruction Paper, subject. Geometrical 
Drawing. 

40. The first eight figures of this plate show the eonven- 
ti >nal methods of representing screws. The actual projection 
of a screw thread will be similar to the projection of the helix 
shown iri Plate V ; but, in order to save the time required to 
locate the points and trace in the curves, the following methods 
arc universally used, except, perhaps, in the case of screws of 
very large diameter and pitch, drawn full size. 

Fig. 1 represents a single square-threaded screw H 
inches in diameter, and f inch pitch. To draw the screw, firsl 
draw the center line m n, and a line .4 B at right angles to it. 
Make the distance AB equal to the diameter of the screw. or 
H inches, and through the points A and B draw lines A D and 
BE parallel to the center line mn. Also lay off on the line 
.1 6 distances A F and B G equal to one-half of the pitch, and 
through the points F and draw lines F II and Q I parallel to 
the center line //* //. These 1 lines show the depth of the thread. 
On the line AD. layoff the width of the thread and of the 
groove, AC, <'■!.. IK. etc., each equal to one-half of the pitch, 
; X i = A of an inch. Draw the line /> ('. and through 
the points J. K. L. M. etc. draw lines parallel to B C. Draw 
faint pencil lines through the points Oand /'../and Q, A' and A\ 
etc., to represent the back edges of the threads, and make tie- 
parts which are seen full lines; then draw the Hues T V. (' IP. 
etCj The method of drawing the remainder of the screw, and 



MECHANICAL DRAWING. 



the reason for using the heavy shade lines, as shown, should be 
apparent without further explanation. 

It will be noticed that the width of the thread and of the 
groove, measured parallel to the center line m n, and the depth 
of the thread, are all exactly the same ; that is, they are each 
equal to one-half of the pitch. If a section were taken through 
the center line run, the thread and groove would look like 
Fig. 71, a series of squares ; hence the term square thread. 

Fig. 2 shows a double square-threaded screw 1^ inches 
in diameter, and with f of an inch pitch. The reason for 
using a double thread is, that if the single square thread were 
used, the depth would be so great as to weaken the bolt or rod 
on which it was cut, and render it unsafe for the 
purpose for which it was intended. To prevent 
this, either the diameter of the rod must be 
increased or the thread must be cut of the same 
depth and thickness as a thread of half the pitch, 
or, in this case, as if the pitch were f of an inch 
X \ — "I °f an inch, as in the preceding problem ; 
another thread of the same size and pitch (f of 
an inch) must be cut half-way between these first 
threads, thus giving a double thread. The pitch, 
or distance that the screw would advance in one 
turn, would be f of an inch, the same as if it were 
a single- threaded screw of f of an inch pitch, 
while the depth of the thread is only half as great. 
To draw it, proceed exactly as in the last figure. To 
get the direction of the line B C, which in this figure represents 
the projection of the bottom edge of the top of the thread, lay 
off A C equal to one-half of the pitch, or f X \ = f of an inch, 
and draw the line B C. The width of the threads and grooves, 
and also the depth of the threads, is one-fourth of the pitch, or 
f X | = ye of an inch. 

Through the points K, L, M, etc. draw faint pencil lines K X, 
etc., to represent the back edges of the threads, and make those 
parts which are seen full lines. Through the point K draw a 
faint pencil line K R, at right angles to the center line m //. 
intersecting the line FH in a, and draw the line Ta, which 




MECHANICAL DRAWING. 5 

represents the bottom of .the thread. The remainder of the 
screw should now be drawn without any trouble. 

Fig. 3 is a single V-threaded screw 1^ inches in diam- 
eter and- having 7 threads to the inch ; that is, the pitch is 1 
of an inch. Draw a cylinder 1^ inches in diameter, having 
in n for the center line. Lay off A B, B C, CD, etc., each equal 
to the pitch, or \ inch. Do the same on the left-hand side. 
By the aid of the T square and 60° triangle make the angles 
A OB, FO'G, etc. The rest of the thread can be drawn by 
referring to the figure. 

Fig. 4 represents a screw exactly like the preceding one. 
except that the thread is left-handed, instead of right-handed as 
in the previous case. 

To ascertain whether a thread is left- or right-handed, hold 
the screw in such a position that its axis is horizontal. If the 
thread is right-handed, as it usually is, the angle which the 
edge of the thread makes with the horizontal on the right-hand 
side is obtuse ; if left-handed, it makes an acute angle with the 
right-hand side of the horizontal. No further instruction 
should be necessary for drawing the thread. 

Fig. 5 represents a double Y-tlireaded screw 1^ inches in 
diameter. It has 3-|- threads per inch ; that is, the pitch is 
1 inch -f- 3-^ = f inch. The same remarks regarding the draw- 
ing of it apply here that were used in describing Figs. 2 and .'5. 

Fig. 6 represents a section of a brass nipple. When the 
diameter of a nipple is given, the inside diameter is always 
meant, unless otherwise specially stated. The actual diameter 
of a nipple or pipe is very rarely given, but must be taken from 
printed tables. The nominal diameter of the nipple shown in 
the figure is 1 inch, but the actual inside diameter is 1.05 
inches; from the table, the outside diameter is found to be L.32 
inches, making the thickness .135 of an inch. Owing to the 
thinness of the shell, pipe threads are finer than the threads on 
the same-sized rods. The coarsest pipe thread i> 8 threads per 
inch. The number of threads per inch on the nipple shown is 
11^. The thread is tapered to make a tight fit, and the length 
of the threaded part on each end is 0.52 0.53 1.05 inch.-, 
of which length the distance between a and /> represents the 



6 MECHANICAL DRAWING. 



perfect thread, while from b to c the thread is chamfered ; that 
is, it dies out gradually. To draw the nipple, make a sectional 
view as shown. Draw a cylinder, having m n as a center line 
1.32 inches in diameter; lay off the inside diameter equal to 
1.05 inches, and draw A B and CD. Now lay off the diameter 
HK equal to 1.26 inches. Then lay off the distances HJ and 
KL, equal to 0.52^0.53 = 1.05 inches; join the points IT 
and J, and K and i, by straight lines representing the top of 
the threads. Now on the center line m n. or on any line 
parallel to it, lay off a distance of 1 inch from the line A C 
downwards. Divide this distance into 11^ parts by means of 
the method given in problem 6, Art. 31. Project the points 
just found upon HJ, and, by means of the T square and 60° 
triangle, draw the threads from IT to J as though all the threads 
were perfect. Draw the threads on KL in the same manner, 
remembering that the divisions on HJ are to be advanced half 
a thread, as shown ; that is, the top of one thread and the bot- 
tom of the preceding thread on the other side will be on a 
horizontal line. Now lay off the distance ah equal to 0.52 
inch, and project it on lines drawn parallel to K L and HJ. 
and touching the bottom of the threads. From the points of 
intersection draw straight lines to /audi, in order to obtain 
the bottoms of the imperfect threads extending from b to c. 
Complete the rest of the drawing in the same manner. 

Fig. 7 shows another method of representing a Y-threaded 
screw. This method has the advantage of making a neat-look- 
ing, drawing, and of being very rapid in delineation. The pitch 
is laid off as in the three preceding figures. The heavily shaded 
lines represent the bottom of the thread, and their lengths are 
determined by constructing an equilateral triangle on the pitch 
distance as shown, and limiting the line to distances between 
two corresponding vert exes of the triangle. The diameter of 
the screw is 1 inch, and the number of threads per inch is 8. 

Fig. 8 represents the same screw shown in Fig. 7, but the lines 
indicating the bottom of the thread are left out altogether. 
This method is used on drawings where haste is necessary. 
Unless in very much of a hurry, the method shown in Fig. 7 is to 
be preferred. Ordinarily, when drawing screws as represented 



MECHANICAL DRAWING. 



by Figs. 7 and 8, it is not customary to lay oft' the pitch and 
the depth of the thread as above mentioned — the distances 
between the lines representing the threads are simply gauged 
by the eye : practice will enable this to be done very quickly 
and accurately. 

Fig. 9 shows two views of a small hand -wheel. To draw 
it. locate the center 0, and through draw the center lines tv 
and in a at right angles to each other. From as a center, 
with the compasses set to the radius of the wheel, or oj- indie-. 
draw the outer circle of the rim ; then, lay off the thickness of 
the rim. which is § of an inch, and draw the inner circle. 
Through 0. with the T square and 60° triangle, draw the center 
lines. IB and CD. With as a center, draw two circles, one 
having a diameter of f of an inch, to represent the hole, and the 
other a diameter of 1} inches, to represent the outside of the 
hub. To draw the arms, make the chords of the arcs a b, c d, 
etc. each }> inch long. Make the chords // /. il\ etc. each § inch 
long, and draw lh. h a, kd, etc. With a radius equal to J of an 
inch, describe the fillets, or are.-. A' B'. < " D' . etc tangent to the 
arms at A'. B' . <" . etc. With a radius equal to \ of an inch. 
describe the fillets or arcs tangent to the inside of the rim and 
to the arms. All of these arcs terminate at the point of 
tangency. The cross-sectioned part on the arm indicates that a 
cross-section taken at EF would look as shown, that is. that the 
arm is elliptical. 

The other view shows a conventional method, largely used in 
drawing rooms, of indicating a section of the wheel. t It i- 
termed a conventional method because it would really be 
impossible to obtain a section like the one shown. Theoret- 
ically, the arm in this view should be sectioned, but. for con- 
venience, a section is imagined to be taken through the rim and 
hub on the line tv, and the two arms are shown in projection 
as they appear on the line/////. Draw the center line /<</ and 
the sections of the rim. as shown. Draw the arm- from the 
dimensions given. Draw the hub as shown, using a radius of 
,"'._, of an inch to draw the fillets or arcs tangent to J /> and to 
the arm: make 0' IV equal to Oil in the other view, and 
describe an arc through //' tangent to the two arcs just drawn. 



MECHANICAL DRAWING 



which are tangent to A B and the arm. The rest of the draw- 
ing can be completed without further explanation. 

Fig. 10 shows a crank, which should be drawn without 
difficulty from the dimensions given. The pin is forced in, 
and the end riveted over at A B, to prevent it from being- 
pulled out. 



SECTION EIXES. 

47. The method ordinarily used for indicating the different 
materials by means of section lines is shown in Fig. 72, in a 
series of small squares drawn according to the usual method of 
sectioning the material named at each square. 




Cast iron, as mentioned before, is indi- 
cated by a series of parallel lines equally 
distant apart. 




Wrought iron is sectioned in the same 
manner, except that every alternate line is 
shaded. 




Steel is sectioned by drawing two lines 
close together, and the third line about H 
times as far from the second as the second 
is from the first, and repeating. 




Brass is sectioned like cast iron, except 
that every other line is broken. 



Fig. 72. 



MECHANICAL DRAWING. 




Babbitt metal is sectioned like cast iron 
— but in both directions, forming little 
squares. 



Wood is sectioned by a series of rings 
and radiating lines like the cross-section of 
an oak tree. 



Fig. 



SCALES. 

48. When it is desired to make a drawing other than full 
size, special scales are used. Thus, suppose it is required to 
make the drawing J size ; then, 3 inches on the drawing would 
represent 1 foot on the object. Hence, if 3 inches are laid oft' 
and divided into 12 equal parts, each of these parts will repre- 
sent 1 inch on the object. If these parts be subdivided into 
2. 4, 8, etc. parts, each will represent h, ^-, J-, etc. of 1 inch 
on the object. A scale of this kind is called a quarter scale, 
or a scale of :* inches to the foot. An eighth scale, or a 
scale of i :> inches to the foot, would he constructed in the 
same way. except that \\ inches would he laid oft instead of 
3 inches. These scales are written 3" = 1 ft., IV' = 1 ft. 

Fig. 73 Bhows the scale (12 inches long) with which the 
student should be supplied, and the manner of using it. One 
edge is divided the whole length into inches and sixteenths of 
an inch, and is used for full-size drawing-. The other edge 
contains two scales, namely. H inches and 3 inches to the loot, 
indicated by the figures H and 3 at .! and B. It will he 
noticed that on the scale of 3 inches to the foot the zero mark 
is at C instead of at a. as in full-size Bcales. It i- put there for 
convenience in reading feet and inches at the same time. The 
figures indicating the number of fret on this -rale are placed 
along the extreme upper edge at P. F.. and /•'. To lay off 2 feet 



10 



MECHANICAL DRAWING. 



*1 



v n 



3f inches from some point, as H, towards an- 
other point, as 7, place the scale on the point 
Hj so that the 2-foot mark will fall on this 
point ; then, from the zero mark, lay off 3f 
inches, as shown, locating the point 7. HI will 
be 2 feet 3f inches long. The same remarks 
apply to the other scale of 1^ inches to the 
foot. To draw to half size, or 6 inches to the 
foot, use the full-size scale, and remember that 
every \ inch on that scale corresponds to 1 
inch on the object, that is, that every dimension 
is only half of the real length. To lay off 5-J 
inches, lay off 5 half-inches and y^- of an inch 
over ; the result is a line 5-J inches long to a 
scale of 6 inches to the foot. 

If it is desired to draw to a scale of f of an 
inch to the foot, or y 1 ^ size, use the scale of 1^ 
inches to the foot, halving all dimensions, as 
in the previous case of drawing to a scale of 6 
inches to the foot with a full-size scale. It 
sometimes happens that a draftsman is obliged 
to make a scale, when the size of his plate is 
limited and a general drawing of some object is 
desired. By general drawing is meant a com- 
plete view of the object in plan, and also one or 
two elevations. In such' a case one scale may 
be too large to enable the drawing to be made 
on a sheet of the required size; another scale 
may make it too small to show up well. For 
example, an \ scale may be too large, and a y 1 ^ 
scale too small ; a y 1 ^ scale may be just right. 
If the draftsman has no T V scale (that is, a 
scale of 1 inch to the foot), he may make one 
by taking a piece of heavy drawing paper and 
cutting out a strip about the size of an ordinary 
scale, and laying off the inch divisions on it. 
Each division or part will represent 1 foot on 
the object. Divide one of the end parts into 



MECHANICAL DRAWING. 11 



12 equal parts, and each will represent 1 inch on the object. 

Lines indicating half- and quarter-inches may be drawn, if 
considered necessary. 

Fig. 74 shows part of a scale made in this maimer, giving 



JiiiNililii'l'lilil'lil 

$2 9' <H 3 l ©) 



A 



FlQ. 74. 

feet, inches, and half-inches — the quarters, eighths, etc. of an 
inch heing judged by the eye. 



DRAWING PLATE, TITEE : ECCEXTRIC AND 
BRAKE LEVER. 

49. Fig. 1 shows an elevation of an eccentric and its 
strap. The strap is made in two pieces, and bolted together 

with a small space, 3V of an inch wide, between them. Locate 
the point 0, the center of the strap, and draw the center lines 
AB, m n, CC, and DD'. Make the offset 0' one-half of the 
throw of the eccentric, or. J inch, as shown, thus locating 
0' . the center of the eccentric shaft. Construct the rest of the 
view from the dimensions given, noting that the arc K E' and 
F F are concentric with 0, while G G' and II H' are concentric 
with ()'. The part FF' G'G is entirely open, and is made so in 
order to lighten the eccentric. 

Fig. 2 is a section of the eccentric and strap. The section 
is drawn in a conventional manner, it heing all taken on the 
line A />. Fig. 1, except that part of the eccentric between G' 
and F\ where, instead of sectioning, the drawing shows it open 
from /, to /', Fig. 2. as if the section had been taken on the line 
OJ. This attracts attention to the open part of the eccentric, 
and shows it more clearly. 

It will he noticed that the slope of the threads in the sec- 
tional part is the same as for the left-handed screw. The 
thread, however, is right-handed, and the reason for showing it 
in this maimer is that it is the hottom of the thread that i> 



12 MECHANICAL DRAWING. 

being looked at ; that is, the section of a right-handed nut and 
the projection of the bottom of a right-handed screw are the 
same as the projection of the top of a left-handed screw. It 
will also be noticed that the sectional lines on the eccentric 
run in opposite directions to those on the strap. This is 
always done when two different pieces meet, and serves to 
indicate that they are separate pieces. Each piece should be 
sectioned entirely in the same direction, no matter if there is a 
break, as in the present case, between L and P. This shows 
that A B CD E is one piece. The dotted hole at/i, Fig. 1, is an 
oil hole. 

Fig. 3 shows a brake lever drawn to a scale of 3 inches = 
1 foot. Owing to its length being too great to be shown 
entirely on the drawing to this scale, the handle is shown as if 
a piece had been broken out, the dimension line, 4 feet 7 inches, 
giving the distance between the two centers and 0'. The 
lever should be readily drawn from the dimensions given. 
To proportion it properly, where the size of the paper does not 
permit the whole of it to be drawn, proceed as follows : The 
length between the centers is 4 feet 7 inches = 55 inches. 
The width through 0' is 2\ inches, and through 0, 4 inches. 
Hence, 4"— 2\" = If" ; 1.75" = the taper in 55 inches. Mea- 
sure off 0i = say, 2 feet. The width at A may be found as 

follows: -L— = taper in 1 inch. -^- X 24 = .76 inch 
bo bo 

nearly, the taper in 2 feet, 4" — .76" = 3.24" = B 0, or the 

width at A. Now locate the point 0', and from it as a center 

describe a curve 2^ inches in diameter. Draw lines tangent to 

this curve, and parallel to the edges between A and already 

found. 

It should be noticed that the center of the brake lever in the 

left-hand view is not situated at the joint where the two parts 

come together, but coincides with the center of the handle, as 

it should do. 

50. Fig. 75 shows the ordinary conventional method of 
representing a nut. The bottom of the thread is 1|- inches in 
diameter, and is represented by the dotted circle ; this shows 



MECHANICAL DRAWING. 



13 



that it is intended for a screw 1-i- inches in diameter. The 
height of the nut equals the diameter of the bolt or screw on 
which the thread is cut, The two views on the center line mn 
should be drawn without difficulty. To draw the curves ea 
and a d, project b and c at right angles to t v in the points 
dj a, and e ; pass arcs of circles through e and a, and through 
a and <l tangent to fg, finding the centers of these arcs by 
trial. The best way of doing this is to draw lines parallel to 
t r midway between e and a and between a and d. Then, by 
trial with the compasses, find a center on these lines such that 
an arc struck with the compasses from this center will pass 







1 
















1 • ! 






. 


21" 





- 





Fig. 



through e and a (or a and d ) and be tangent to fg. In the 
right-hand view, the radius of the arc be is the same as the 
height of the nut ; the centers of the other two arcs are found 
by trial in the manner just described. 



DRAWING PliATE, TITLiE : SHEAVE. 

5 1 . This plate shows a drawing of a sheave 1 feet 5 inches 

in diameter, used for transmitting power by rope. In order to 



14 MECHANICAL DRAWING. 



use as large a scale as possible, only half of the side elevation is 
shown. This is all that is necessary, since the other half is 
precisely the same. In the lower view, the right half is shown 
in section, and the left half in plan or projection ; in other 
words, but three-quarters of the sheave is shown, the other 
quarter is imagined to be removed. This method of represent- 
ing an object is frequently used in drawing rooms, and saves 
not only space, but the time that would be required to complete 
the drawing. The fourteen arms of the sheave are made of 
wrought iron, the ends being upset and grooved as shown in the 
section. When casting the sheave, the arms are laid in their 
proper positions in the mold, the rim and hub being cast 
around them. The metal flows around inside the grooves and 
thus prevents the arms from being pulled out, It will be 
noticed that while one arm extends from the rim to the upper 
end of the hub (as viewed in the lower figure), the one next to 
it extends to the lower end ; the next to the upper end again, 
and so on. This arrangement makes a stronger sheave than if 
all the arms extended to the center. 

Having drawn the principal center lines, construct the 
side elevation. The character of the curves at A, B : etc. may 
be more clearly understood by referring to the sectional view. 
This shows that they are the projections of the intersection of 
the bosses, through which the arms enter, and the inside of the 
rim which is made rounding. 

These curves may be constructed by finding different points 
on them, but, for practical purposes, the student may imitate 
those shown as nearly as he can. The dotted semicircle in the 
rim represents, of course, the bottom of the groove. The sec- 
tional part of the lower view should now be drawn. Lay off 
C = C = 4 feet 5 inches -^2 = 2 feet 2. 5 inches. Lay 
off a distance of 2 inches on each side of m n and through the 
points thus obtained draw lines parallel to m a and cutting 
the projection line L L' in D and D'. These lines represent the 
outer edges of the rim. With a T 3 g-inch radius describe arcs 
tangent to the outer edges of the rim and to the line D D'. With 
a radius equal to \\ of an inch, and a center on m n, describe an 
arc passing through C. With a radius of 6 inches describe arcs 



MECHANICAL DRAWING. 15 



tangent to the arc through C, and also to the small arcs already 
drawn. The centers #', 0" are found by trial. With Cas a 
center, and a radius equal to \\ inches, describe the are which i< 
shown ] tartly dotted ; this, of course, represents the inside of the 
rim. With but few exceptions, all dotted lines have been 
omitted from the drawing, and only those parts actually seen, 
have been shown. This was done to prevent the student from 
becoming confused by a multiplicity of lines. Draw the outline 
of the entire hub from the dimensions given. Draw the center 
lines / /and t' r\ 3§ inches on each side of and parallel to /;/ n : 
these two lines contain all the points of intersection of the 
center lines of the arms with the hub. Draw the arm G from 
the dimension given. Project the centers of the bosses J, H, 
and I upon the center line mn, locating the points J 2 , H. z , and 
/,. and construct the projections of the bosses themselves. 
Project J' upon tv, in J 1T Join J Y and J 2 by a straight line ; 
J { ,/, will be the center line of the projection of the arm ,/',/. 
Project H' upon t x i\, in H xi and draw H x H,. Draw f x f 2 in a 
similar manner. Finish the sectional view. 

The hub is cored out to a diameter of 6^ inches, the recess 
thus formed being 4 inches long. 

The other half of the sheave, which is shown in projection, 
should be easily drawn without further explanations. 

The center lines of the arms /' /, J,/,, H' H, etc. may be 
omitted from the finished drawing, as it is not customary to ink 
them in on shop drawings ; the dotted projection lines /'/,. 
H' H l , J' ' ,/,, etc., may also be omitted for the same reason. 



DRAWING P^ATK, TITLE: REVERSING LEVER. 

52. This plate shows a drawing of a reversing lever, 
together with its stand, which is suitable for a brake lever on 
hoisting-engines. To draw it, first draw the center line m n. the 
base line A B y and the base and bosses in the side elevation. 
Locate the center on mn, V inches below A B. With as a 
center, and a radius equal to 2 feet 3 inches, describe an arc of 



16 MECHANICAL DRAWING. 



a circle. On each side of m n lay off 12 inches, and draw ver- 
tical lines through the points ; these lines intersect the arc just 
described in 0', 0', the centers of the pins. The arc intersects 
mn in P ; hence, lay off 1 inch on each side of P, along m n, 
and draw arcs of circles through these points ; they will be the 
limiting lines of the top and bottom of the wronght-iron sector. 
With the two centers O'and a radius of 1 inch, describe the two 
circles as shown. Set the spacing dividers to 1 inch, and step 
off the teeth on the outer circular arc. Draw all the short 
shaded lines, as a 6, radial ; that is, if a line be drawn from the 
point a to the center 0, the point b will fall on this line. 
Measure, from the base line upwards, CD = 7-J- inches, and, 
with a center on m n and a radius equal to 10 inches, pass an 
arc through the point D ; also one through E, half an inch 
below D, using the same center. With a radius equal to 18 
inches, find by trial the center Q, which is so located that an 
arc described with this point as a center, with a radius 18 
inches long, will be tangent to the arc just passed through D, 
and to the circle described about 0'. With the same center, 
and a radius ^ of an inch longer, describe the other arc tangent 
to the arc through E. Now draw a faint pencil line on each 
side of, and parallel to, mn, and distant 5f inches from it. 
With E as a center, and a 3-inch radius, describe an arc tangent 
to the perpendicular just drawn, and to the upper edge of the 
base. Find by trial the center Q', such that an arc described 
w T ith Q' as a center, and a radius equal to 19f -f ^ = 19f inches, 
will be tangent to the arc just drawn and to the circle whose 
center is 0'. With the same center, and a radius equal to 19f 
inches, describe another arc tangent to an arc described with R 
as a center and a radius equal to 3^ inches. The construction 
is the same for each side of m n. 

The remainder of this view can be easily drawn from the 
dimensions given. The dotted part between C and is a rib ; 
it is shown more clearly in the front elevation, in which F G is 
its outer edge. The line c de on the top of the lever indicates a 
flat surface. In the front elevation the line HI represents the 
line described with the 19f-inch and 3|-inch radii, and the line 
H' T represents the line described with the 18-inch and 10-inch 



MECHANICAL DRAWING. 17 

radii. Theoretically, .the entire rib FGKL should be sec- 
tioned, and the outline of the sectional part between N' and N 
should be dotted, but, for convenience, a conventional method 
is used. The thickness, f inch, of the sectional part, is the 
thickness of all that part of the arm below the base, except that 
included between the dotted lines on the side elevation. The 
dotted line RS is the projection of the edge R S'. 



DRAWING PLATE, TITLE: FRICTION CLUTCH. 

53. This plate shows a cross-section of a friction clutch.. 

The pinion B has a long bearing, and is loose on the shaft S, 
which runs continually. The brake wheel A is keyed to the 
huh of the gear-wheel B in such a manner that when A revolves, 
B revolves also. The clutch (', which has pieces of wood 
inserted in its periphery, is keyed to the shaft S, and revolves 
continually with the shaft. The sleeve D is a loose fit on the 
shaft, and can be moved back and forth by means of a lever or 
shifter having at its end the collar E E. A feather, not shown, 
forces the sleeve to turn with the shaft. When D is thrown 
inwards in the direction of the arrow, the pieces FF&re forced 
outwards. This causes the clutch holding the pieces of wood 
before mentioned to be forced out likewise. The wood presses 
against the inside of the brake wheel A, and the friction 
between the two surfaces causes A to revolve, at the same time 
turning the pinion B, which engages with ;i gear-wheel not 
shown. 

It will he noticed by the cross-sectioning that the part of the 
collar E which is shown is made of wrought iron ; that there 
are two wrought-iron collars 11 and //'. and two steel washers 
'/'and T . The collar H' is secured to the shaft by means of a 
set-screw actuated by a wrench, while the collar // is secured 
by two small screws actuated by a screwdriver, the heads being 
below the surface of the collar. The stcrl washer 7" is riveted 
to the collar K. 

The student should he able to construct the drawing from the 
dimensions given. 



18 MECHANICAL DRAWING. 



DRAWING PEATE, TITEE : BENCH VISE. 

54. This plate shows a bench vise and its details. 
A drawing of this kind is called a detail drawing. Fig. 1 is 
a complete top view, and Fig. 2 is a section through the center 
line CD. The remaining figures are drawings of the different 
parts, or details. The actual practice in the drawing-room 
would he to draw Figs. 1 and 2 first, and then the details, 
but in the present case the student will do w r ell to draw the 
details first, as it will help him in drawing the other two views, 
particularly since nearly all of the dimensions are given on the 
details. Following out this plan, leave space for Figs. 1 and 2, 
and begin by drawing Fig. 3. This consists of three views of 
the jaw, the part marked A in Figs. 1 and 2. The parts marked 
B, C, and Z>, in Figs. 1, 2, and 3, are circular in shape, so as to 
permit the back jaw A of the vise to swing when the pin E is 
removed. This allows the vise to hold tapered pieces with as 
much firmness as straight ones. 

Fig. 4 is a detail, with dimensions, of the pin E. 

Fig. 5 is a detail of that part of the vise marked F in Figs. 1 
and 2, and also of the wrought-iron nut G. The reason that 
only a part of the nut G is sectioned in Fig. 2 is that a con- 
ventional method has been used. In reality, the whole of the 
nut should be sectioned. It would be impossible to take a 
section of the nut in the manner shown. 

Fig. 6 shows a top view, longitudinal section, and cross- 
section of the front jaw. This is cored out to admit the screw 
and the nut. The form of the cored part is shown by the 
longitudinal section and cross-section through EF and A B. 
The front jaw moves in and out with the screw, as shown by 
Fig. 2, the collar L being held in place by the small set-screw. 
This is sufficient, since there is no stress on the collar beyond 
the force required to pull the jaw outwards, the force exerted 
by screwing the jaws together coming entirely on the surfa.ce M 
of the screw head. 

A separate detail drawing of the screw is not required, since 
all the dimensions are given in Fig. 2. Figs. 1 and 2 can now be 
drawn, the necessarv dimensions being obtained from the details. 



MECHANICAL DRAWIN(i. 19 



For ordinary shop drawings, the details are usually drawn to 
a larger scale than the general drawings. Figs. 1 and 2. In this 
case the size of the plate would not permit an enlargement. 



TRACINGS. 

55. In actual practice in the drawing room, it is necessary 
to have more than one copy of a drawing. It would be very 
expensive to make a finished drawing every time an extra 
copy was wanted, and. to avoid this, tracings and blue-prints arc 
made. Any number of 1 due-print copies can be made from the 
same tracing. A complete pencil drawing is made first ; then, 
instead of inking in an heretofore, a piece of tracing paper or 
tracing cloth of the same size as the pencil drawing is fastened to 
the board over the original drawing. The tracing paper or 
cloth being almost transparent, the lines of the drawing can be 
readily seen through it. and the drawing is inked-in on the 
tracing paper or cloth in the same manner as if inking-in a 
finished drawing. 

Tracing paper is hut little used in tins country. It is 
easily torn, and cannot he preserved so well as tracing (doth. 
The two sides of the tracing cloth arc known as the glazed 
side and the dull side ; they are also known as the front and 
the hack. The glazed side, or front, is covered with a prepara- 
tion that gives it a very smooth, polished surface ; the back, or 
dull side, has very much the appearance of a piece of ordinary 
linen cloth. Either side may he used for drawing upon. bu1 
when the glazed side is used, care must he taken to remove all 
dirt and grease, otherwise the ink will not How well from the 
pen. This can he done by taking a knife or a tile and scraping 
or filing chalk upon the tracing cloth : then, take a soft rag of 
some kind — cotton flannel or chamois -kin — and rub it nil over 
the tracing cloth, being sure to rub chalk over every -pot. 
Finally, dust the rag and remove as much of the chalk from 
the cloth as can he gotten off by rubbing with the rag. The 
finer the chalk powder is. the better. It is not usual to chalk 
the dull side, bul it improve- it to do so. The glazed side I 



20 MECHANICAL DRAWING. 



ink much better than the dull side, the finished drawing looks 
better and will not soil so easily, and it is also easier to erase a 
line that has been drawn on this side. Pencil lines can also be 
satisfactorily drawn on the dull side, and, if it is desired to 
photograph the drawing, it is better to draw on this side. The 
draftsman uses either side, according to the work he is doing, 
and to suit his individual taste, but if the glazed side is used, it 
must be chalked. The tracings are drawn in a manner similar to 
the finished drawings, the center lines, section lines, etc. being 
drawn exactly as previously described. 

After having drawn the plate entitled Bench Vise, the student 
should make a tracing of the plate entitled Friction Clutch. 



BI/LE-PRIXTIl^G. 

56. Blue-printing is the process of duplicating a tracing by 
means of the action of light upon a sensitized paper. The follow- 
ing solution is much used for sensitizing the paper : Dissolve 
2 ounces of citrate of iron and ammonia in 8 ounces of water ; 
also, 1^ ounces of red prussiate of potash in 8 ounces of water. 
Keep the solutions separate, and in dark-colored bottles in a 
dark place where the light cannot reach them. Better results 
will be obtained if \ an ounce of gum arabic is dissolved in each 
solution. 

When ready to prepare the paper, mix equal portions of the 
two solutions, and be particularly careful not to allow any more 
light to strike the mixture than is absolutely necessary to see 
by. For this reason, it is necessary to have a dark room to 
work in. There must be in this room a tray or sink of some 
kind that will hold water ; it should be larger than the blue- 
print, and about 6 inches deep. There should also be a flat 
board large enough to cover the tray or sink. If the sink is 
lined with zinc or galvanized iron, so much the better. There 
must be an arrangement like a towel rack to hang the prints on 
while they are drying. For the want of a better name, this 
arrangement will be called a print rack. The paper used for 
blue-printing should be a good, smooth, white paper, and may 
be purchased of any dealer in drawing materials. Cut it into 



MECHANICAL DRAWING. 21 

sheets a little larger than the tracing, so as to leave an edge 
around it when the tracing is placed upon it. Place eight or 
ten of these sheets upon the flat board before mentioned, taking 

care to spread flatly one above another, so that the edges do not 
overlap. Secure the sheets to the hoard by driving a brad or 
small wire nail through the two upper corners sufficiently far 
into the board to hold the weight of the papers when the board 
is placed in a vertical position. Lay the board on the edges of 
the sink, so that one edge is against the wall and the board is 
inclined so as to make an angle of about 60° with the horizontal. 
Darken the room as much as possible, and obtain what light 
may be necessary from a lamp or gas jet. which should he 
turned down very low. With a wide camel' s-hair brush or a 
fine sponge, spread the solution just prepared over the top sheet 
of paper. Be sure to cover every spot, and do not get too much 
on the paper. Distribute it as evenly as possible over the 
paper, in much the same manner that the finishing coat of 
varnish would be put on by a painter. Remove the sheet by 
pulling on the lower edge, tearing it from the nail which holds 
it. and place it in a drawer where it can lie flat and be kepi 
from the light. Treat the next sheet, and each succeeding sheet, 
in exactly the same manner, until the required number of 
sheets has been prepared. 

Unless a large number of prints is constantly used, it i- 
cheaper to buy the paper already prepared. It can be bought 
in rolls of 10 yards or more, of any width, or in sheets already 
cut and ready for use. There is very little, if anything, saved 
in preparing the paper, and better results are usually obtained 
from the commercial sensitized paper, since the manufacturers 
have machines for applying the solution, and are able to dis- 
tribute it very evenly. 

,")7. In Figs. 76 and 77 are shown two views of a printing 
frame which is well adapted to sheets that arc not over 
17" 21". The frame is placed face downwards, and the 
hack J is removed by unhooking the brass spring clips />. I> 
and lilting it out. The tracing is laid upon the gla>s C, with 
the inked side touching the glass. A sheet of the prepared 



99 



MECHANICAL DRAWING. 



paper, perfectly dry. is laid upon the tracing with the yellow 
(sensitized) side downwards. The paper and tracing are 
smoothed out so as to lie perfectly flat upon the glass, the cover 
A is replaced, and the brass spring clips B. B are sprung under 
the plates D, so that the back cannot fall out. While all this 
is being done, the paper should be kept from the light as much 
as possible. The frame is now placed where the sun can shine 
upon it. and adjusted as shown in Fig. 77, so that the sums rays 
will fall upon it as nearly at right angles as possible. Accord- 
ing to the conditions of the sky — whether clear or cloudy — 




Fig. 



and the time of the year, the print must be exposed from 3 to 
15 minutes. The tray, or sink, already mentioned, should be 
filled to a depth of about 2 inches with clear water (rain water 
if possible). The print having been exposed the proper length 
of time, the frame is carried into a dark part of the room, the 
cover removed, and the print (prepared paper) taken out. Now 
place it on the water with the yellow side down, and be sure 
that the water touches every part of it. Let it soak while 



MECHANICAL DRAWING. 



23 



putting the next print in the frame. Be sure that the hands are 
dry before .touching the next print. The first print having 
soaked a short time (about 10 minutes) take hold of two of its 
opposite corners, and lift it slowly out of the water. Dip it 
back again and pull out as before. Repeat this a number of 
times until the paper appears to get no bluer ; then hang it by 
two of its corners to dry on the print rack above spoken of. If 
there are any dark purple or bronze-colored spots on the prints. 
it indicates that the prints were not washed thoroughly on 
those spots. If these spots are well washed before the print i> 
dried, they will disappear. 




Fi<;.77. 



58. It is best to judge of the proper time of exposure to the 
light by the color of the strip of print projecting beyond the 
edge of the tracing. To obtain the exact shade of the projecting 
edge, take a strip of paper about 12 or 14 inches long and 3 or 
4 inches wide. Divide it into, say, 12 equal parts by lead-pencil 
marks, and, with the lead pencil, number each part 1. 2, .'}. etc. 
Sensitize this side of the paper, and, after it has been properly 



24 



MECHANICAL DRAWING. 



dried, place it in the print frame with the sensitized side and 
the marks and figures against the glass. Expose the whole 




strip to the light for one minute ; then cover the part of the 
strip marked 1 with a thin board or anything that will prevent 
the light from striking the part covered. At the end of the 



/ 



MECHANICAL DRAWING. 25 



second minute, cover parts 2 and 1 : at the end of the third 
minute, parts 3, 2. and 1, etc. When twelve minutes are up, 
part 1 will have been exposed one minute: part 2. two minutes, 
etc.. part 12 having been exposed twelve minutes. Remove 
the frame to a dark part of the room, and tear the strip so as to 
divide it into two strips of the same length and about half the 
original width. Wash one of the strips as before described, 
and when it has dried, select a good rich shade of blue, neither 
too light nor too dark; notice the number of the part chosen, 
and it will indicate the length of time that the print was 
exposed. Examine carefully the corresponding part of the 
other strip, and the correct color of the edge of the print pro- 
jecting beyond the tracing is determined. All prints should be 
exposed until this color is reached, no matter how long or how 
short the time may be ; then they should be immediately taken 
out and washed. 

In Fig. 78 is shown a patented frame which can be shoved 
out of the window and adjusted to any angle. When not in 
use. it can be folded up against the wall, and occupies but 
little space. It is made in different sizes from 16" X 24" to 
48" X 72". It is one of the best frames in the market, and is 
placed in such a position relatively to the window that the 
window can be lowered to the to]) of the main arm. when it is 
desired to keep out the cold during the winter. 



/ 



LIBRARY OF CONGRESS 



019 971 086 2 



/ 



LIBRARY OF CONGRESS 



019 971 086 2 



